This invention is related to gas sensing elements useful in sensing the concentration of a gas in a medium, and to methods for making such sensing elements. In particular, the invention relates to gas sensing elements useful in sensing the concentration of a gas, for example, carbon dioxide, in an aqueous medium, for example, blood.
In many situations, it is useful to determine the concentration, e.g. partial pressure, of a gas in a medium, e.g., a fluid medium. One such situation is the determination of the concentration of gas, e.g., carbon dioxide, oxygen and the like, in blood. Substantial recent interest has been focused on performing such determinations on a real time basis so that the current status of the patient being treated can be effectively monitored. Fluorescence-based sensors have been utilized to accomplish real time blood gas sensing. For example, fluorescence-based sensors can be used in an extracorporeal blood loop as shown in Cooper U.S. Pat. No. 4,640,820 and in vivo as disclosed in Lubbers et al Reissue U.S. Pat. No. 31,879. Each of these patents is incorporated by reference in its entirety herein.
Yafuso et al U.S. Pat. No. 4,824,789 discloses a sensing composition and sensor useful for determining the concentration of a gas in blood. This patent discloses a sensing composition which is useful when placed on the optical surface of an optical fiber to form a sensor. The sensing composition comprises an aqueous first phase dispersed in a cross-linked polymeric second phase. The aqueous first phase includes a dye, for example a pH sensitive dye. The cross-linked polymeric second phase, which is gas permeable and ion impermeable, preferably is silicone-based includes a hydrophobic filler, such as hydrophobic fumed silica particles. These sensing compositions may further include one or more emulsification enhancement agents, such as water soluble dextran and polyvinyl alcohol. This patent discloses that a teflon sleeve is used on the optical fiber during manufacture of the sensor to retain the sensing composition precursor on the optical surface of the optical fiber. The sensor also includes an opaque overcoat, such as cellulose impregnated with carbon black, to optically isolate the sensing composition.
The use of disposable cassettes for blood analysis is of substantial current interest, for example, to eliminate cross-patient contamination and to keep more expensive components, e.g., optical and electronic components, of the sensor system from being exposed directly to blood. One disposable cassette system is disclosed in commonly assigned U.S. patent application Ser. No. 229,617, filed Aug. 8, 1988.
Such a disposable cassette includes an indent, well or cavity into which a sensing element is at least partially placed. The sensing element is exposed to blood and the sensing component, e.g., a fluorescent dye, gives off a signal which varies in response to variations in the concentration of the gas of interest in the blood. A signal transmitter, e.g., an optical fiber, spaced away from the cassette well and blood transmits this signal to a processor where it is analyzed to provide the desired blood gas concentration determination.
One problem which has presented itself with regard to the use of these cassettes is that of inconsistent and/or inaccurate blood gas concentration determinations. For example, such cassette blood gas sensors have exhibited "drift"; that is, the signal representing a certain given gas concentration varies or drifts over time. Also, when the sensing element is manufactured or assembled in situ, e.g., in the well of the cassette, there is no chance to screen the sensing element before it is bonded to the cassette. Thus, non-specification sensing elements can be included in the cassette and determined as being non-conforming only after the assembled cassette, which often includes a number of other sensing elements for different blood constituents, is tested. At this point, the entire cassette must be discarded, resulting in substantial waste and cost.
It would be clearly advantageous to provide new sensing elements and/or new methods for making sensing elements, particularly for use in such cassettes.